Abrasion-resistant, infrared transmitting optical components

ABSTRACT

A body resistant to abrasion and transmissive to light is provided which is formed essentially of zinc sulfide or zinc selenide having incorporated therein small amounts of tellurium. The body can form a layer over zinc sulfide, zinc selenide, or a conventional substrate such as glass. The substrate is thereby protected against fracture and erosion by environmental forces, such as high velocity impact with rain or other atmospheric particles, encountered by aircraft at high speed and over extended periods of time.

BACKGROUND

Transparent exterior partitions of aircraft must be durable enough tosecure operators and equipment from harsh environmental conditions. Inaddition, to enable operation of sensitive equipment employed inreconnaissance, such as is used for infrared imaging systems and forweapon delivery applications, these materials must be transmissive tolight in a spectrum ranging from visible to infrared wavelengths.

Existing exterior partitions include barriers, such as largemultispectral windows. Such windows often limit system design in FLIR(forward looking infrared) and infrared imaging systems. The inabilityof these windows to withstand rain abrasion has been a continuingproblem for many years with no satisfactory solution still in sight.

Aircraft typically impact raindrops at high speed, necessitating anexternal barrier which is sufficiently strong to resist fracture.Raindrops also create minor indentations on the outer surface ofconventional windows, imparting distortions to the optical quality ofthe partition and deleteriously affecting operators vision and theperformance of equipment. In addition to strength and hardness, thesematerials must be resistant to rain or particle erosion caused byextended exposure in use of aircraft over long periods of time.

Thus, the materials of construction must be hard enough to withstand theabrasive effect of raindrops impacting transparent surfaces at highspeeds. Further, increased hardness and strength of known materials mustbe accomplished without diminishing optical transmissivity.

A solution to problems of strength and hardness of opticallytransmissive barrier is formulation of materials which provide such acompromise of mechanical properties without deleteriously affectingoptical quality. Equipment and operating systems housed within aircraftare constricted by the limitation of transparent partitions in thefuselage. Windows constructed of such materials must be formed so as toaccommodate particular use requirements.

Attempts to increase the hardness and strength of known materials haveincluded introduction of dopants to a coating material during chemicalvapor deposition of that material onto a window or panel substrate.Common window substrates include glass, germanium (Ge), silicon (Si),zinc sulfide (ZnS), and zinc selenide (ZnSe). Dopants added haveincluded arsenic, aluminum, and mixtures thereof. See, for example,Hardened CVD Zinc Selenide for FLIR Windows, Raytheon Company, ResearchDivision, AFML-TR-75-142, dated Sep. 1975. These efforts have allresulted in unacceptable loss of optical transmissivity (See pages 70-71of the referenced research report).

A need exists, therefor, to preserve the high resolution capability ofsophisticated reconnaissance and weapon delivery systems by increasinghardness, strength and durability of known protective window materialswithout diminishing their optical transmissivity.

SUMMARY OF THE INVENTION

The present invention discloses a method of forming a resultant ZnS orZnSe structure and device which is highly resistant to abrasion withoutsubstantially diminishing the optical transmissivity properties of theconstituent materials. Small amounts of tellurium (Te) are added to theoptically transmissive zinc selenide or zinc sulfide material.Alternatively, a coating is applied to a suitable substrate, the coatingbeing formed of zinc selenide or zinc sulfide which contains smallamounts of tellurium.

In one embodiment, a body is formed of zinc sulfide or zinc selenidewhich contains small amounts of tellurium. The resultant article ishighly resistant to abrasion, such as erosion caused by raindrops, yetis strong enough to withstand impact at high speed with raindrops.

In another embodiment, a coating of zinc selenide or zinc sulfide isdisposed over a substrate. Small amounts of tellurium are incorporatedwithin the zinc sulfide or zinc selenide which hardens the coating forimpact resistance and abrasion resistance without diminishingtransmission of light in the visible and infrared spectrums.

In yet another embodiment, a method and apparatus for abrasionresistance and transmission of visible and infrared light is provided inwhich zinc sulfide or zinc selenide vapor, and tellurium vapor areco-deposited over a substrate to form one or more optical layerscomprising mixtures of zinc sulfide or zinc selenide having smallamounts of tellurium.

The above, and other embodiments of the invention, will now bedescribed, in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of one embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the presentinvention.

FIG. 3 is a sectional view of a third embodiment of the presentinvention.

FIG. 4 is a sectional view of a fourth embodiment of the presentinvention.

FIG. 5 shows measured transmissivity of infrared energy in wavelengthsranging from about 2.5 to 10 micrometers of an optical coating of thepresent invention over a zinc sulfide substrate as is described inExample 2.

FIG. 6 shows measurements of Knoop hardness exhibited by the presentinvention as is described in Example 3.

DETAILED DESCRIPTION OF THE INVENTION

A body of ZnS or ZnSe with small amounts (less than about 2% Te byatomic weight) is formed which is highly resistant to abrasion caused byhigh velocity encounter with raindrops, without substantiallydiminishing the optical transmissivity properties of the ZnS or ZnSematerial. Light energy may be transmitted through the body undiminishedin intensity in the spectrum of wavelengths from visible to infrared.The body is highly resistant to erosion by environmental factors overlong periods of time. The body can be formed as a window to providevisibility from within an aircraft. Because of increased strength andhardness of the present invention, large multispectral windows foraircraft can additionally be formed. Alternatively, the presentinvention can be contoured to form a portion of a fuselage. These bodiesoffer protection for many applications at low cost, in particular, forFLIR (forward looking infrared) and infrared imaging, such as are usedfor reconnaissance and weapon delivery systems.

In one embodiment of the present invention, shown in FIG. 1, opticalbody 10 is formed essentially of zinc sulfide. Small amounts oftellurium are incorporated within the body to form a material which isimpact resistant and erosion resistant to small airborne particles, suchas rain, without diminishing optical transmissivity of the zinc sulfide.Optical body 10 can be formed over contoured surface 12 of a mandrel orgraphite susceptor 14 which is later separated from optical body 10 orburned away. In an alternate embodiment, the optical body 10 is formedof zinc selenide having small amounts of tellurium incorporated within.

It is to be understood that "small amounts," as that term is used hereinto describe the levels of tellurium incorporated with zinc sulfide orzinc selenide, means that tellurium comprises greater than about 0.05%and less than about 2% atomic weight of the zinc selenide or zincsulfide material of the body or optical layer of the present invention.In the preferred embodiment of the invention, tellurium approximates 1%atomic weight of the doped zinc selenide of zinc sulfide.

As can be seen in FIG. 2, the present invention can also be embodied asa protective layer 18 of ZnS/Te or ZnSe/Te over a conventional opticalbody 20, such as glass. In this embodiment, protective layer 18 isformed over an outward surface 22 of the optical body 20 and iscomprised essentially of zinc sulfide or zinc selenide, havingincorporated therein small amounts of tellurium. Protective layer 18thus protects the optical body 20 from impact and from wear caused bycontinuous exposure to harsh environmental elements.

Alternatively, adhesion layer 24 can be disposed between an optical body26 and protective layer 28 for bonding protective layer 28 to opticalbody 26, as is shown in FIG. 3. Protective layer 28 can be comprised ofzinc sulfide or zinc selenide and is hardened by having incorporatedtherein small amounts of tellurium. Thorium (IV) Fluoride (ThF₄) can beadded to protective layer 28 to help bond protective layer 28 toadhesive layer 24. Optical body 26 can be zinc sulfide, zinc selenide,glass or some other conventional transparent or translucent aircraftshielding material. Adhesion layer 24, which is disposed between opticalbody 26 and protective layer 28, can be zinc sulfide or zinc selenide.In an optional embodiment, an external adhesion layer 30, shown in FIG.4, can be disposed over the protective layer 28 and can consistessentially of zinc sulfide or zinc selenide. External adhesion layer 30facilitates adherance of additional coatings or screens which can beapplied over the present invention and prevents formation of atellurium-rich layer distinct from protective layer 28.

In another embodiment, zinc sulfide or zinc selenide is hardened by amethod of the present invention to form a layer which is abrasionresistant without substantially diminishing the optical transmissivityproperties of the ZnS or ZnSe material. Vapors of zinc sulfide or zincselenide and of tellurium vapor are formed by a physical vapordeposition (PVD) process in well known coating chambers, andsubsequently co-deposited on optical body 20 (FIG. 2). Accumulation ofthe vapors co-deposited on the optical body 20 forms protective layer18, comprising zinc sulfide or zinc selenide containing small amounts oftellurium, as seen in FIG. 2.

"Physical vapor deposition", or "PVD" as that term is used here, meansdeposition of vapors generated by molecular beam epitaxy (MBE), electronbeam generation, thermal generation, sputtering or ion beam generation.

In another preferred embodiment of the invention, shown in FIG. 3,adhesion layer 24 and protective layer 28 are deposited on optical body26 by sequential exposure of optical body 26 to vapors, thereby formingadhesion layer 24 and protective layer 28. Optical body 26 is exposed tozinc sulfide or zinc selenide, or mixtures of zinc sulfide or zincselenide vapor with thorium (IV) fluoride vapors. Deposition of any ofthese vapors or mixtures of these vapors, as described, form adhesionlayer 24, seen in FIG. 3. Adhesion layer 24 is subsequently exposed to amixture of tellurium vapor and either zinc sulfide or zinc selenidevapor, which co-deposit on adhesion layer 24, to form protective layer28, comprised of a compound or mixture of ZnS/Te or ZnSe/Te.

As shown in FIG. 4, an external adhesion layer 30 can optionally beformed. Protective layer 28 (as previously described) is exposed to zincsulfide or zinc selenide vapor in a PVD chamber. The vapor is thendeposited on protective layer 28 to form exterior layer 30.

Protective layer 28 can be formed by PVD, wherein ZnS or ZnSe vapors andtellurium vapor are co-deposited on the adhesion layer 24 while theoptical body 26 is brought to an elevated temperature by localizedheating. The optical body 26 is heated by radiant heaters toapproximately 150° C. during the coating steps. Optical body 26 is thenexposed to vapor of zinc sulfide or zinc selenide, provided by anelectron beam source, and of tellurium, which is formed at a temperaturein the range of approximately 290° C. to 340° C. by a thermal source.The vapor mixture is then deposited on the cooler optical body 26 toform a protective layer 28.

The present invention is further described in the following examples,which are not to be considered limiting in any way.

EXAMPLES Example 1

A glass optical body to be coated was exposed to two evaporation sourcesin a PVD chamber, each source was placed behind an independentlyoperated shutter. Zinc sulfide constituted one evaporation source, andevaporation was generated by an electron beam source. The rate of zincsulfide evaporation was controlled by a crystal rate monitor, whichmeasures the rate of crystal deposition. A crystal rate monitor suitablefor use in the method of the present invention is manufactured byLeybold-Inficon, Inc., Model XTC. Tellurium was placed in a refractorymetal heater behind the second shutter and was evaporated from analuminum oxide crucible. The rate of evaporation of tellurium wascontrolled by measuring the temperature of tellurium in the crucible. Athermocouple was placed in a quartz tube which was then placed in thecrucible. Tellurium was then heated behind the shutter and stabilized ata desired temperature. The shutter securing zinc sulfide vapor heated bythe electron beam was opened, exposing the optical body to zinc sulfidevapor. A few hundred angstroms of zinc sulfide was deposited on theoptical body, and then the shutter securing tellurium vapor was thenopened, so that zinc sulfide and tellurium were co-deposited on theglass substrate to form a hardened protective layer. When the desiredprotective layer had been deposited, the tellurium shutter was closedfirst and a few hundred angstroms of undoped zinc sulfide was depositedto form an exterior coating and thereby prevent formation of atellurium-rich layer distinct from the protective layer and which aidsin adhesion of additional layers such as a wire mesh. The zinc sulfideshutter was subsequently closed to conclude deposition of the externaladhesion coating.

The rate of tellurium deposition was controlled by associating a desiredvapor pressure with a given temperature determined by a crystal monitor,on the assumption that the rate of deposition would be proportional to aselected vapor pressure. After a measurable deposition rate which wasassociated with a known temperature, the temperature of the telluriumvapor source was reduced to obtain a desired deposition or "impingement"rate relative to the rate of deposition of zinc sulfide. In thisexample, the tellurium impingement rate was set at a value 100 timeslower than the rate of zinc sulfide deposition, the tellurium sourcebeing approximately 320° C.

Example 2

Example 1 was repeated using zinc sulfide as an optical body. Theprotective layer formed was approximately 1 micrometer thick. FIG. 5represents measured transmissivity of infrared energy in wavelengthsranging from 2.5 to 10 micrometers. As can be seen, within the limits ofmeasurement, there was no adsorption or index change produced by Tedoping.

Example 3

The layer of Example 2 was repeated. The coating being 20 micrometersthick. The resultant material was crack resistant: Knoop hardness wasmeasured and is shown in FIG. 6 relative to a chemical vapor depositionof zinc sulfide optical body and to soda lime glass.

Example 4

Samples of a zinc sulfide optical body coated as in examples 1, 2 and 3were prepared having optical coating thicknesses of 9 micrometers, 18micrometers and 27 micrometers. Erosion tests were conducted on thesethree samples measuring damage caused by 3.5 millimeter diameterraindrops impacted at a 60° angle of incidence at speeds of 1200 feetper second and 1600 feet per second. Vestigal damage was observed on allsamples at 1200 feet per second. Samples retested at 1600 feet persecond showed significant damage. Uncoated zinc sulfide optical bodiesshowed vestigal damage at 700 feet per second in similar tests.

EQUIVALENTS

It is to be understood that gallium arsenide (GaAs), germanium andsilicon substrates are to be included as possible embodiments of opticalbodies which can be coated with hardened ZnS or ZnSe according to thepresent invention. ZnS and ZnSe can comprise protective layers depositedin combination with small amounts of tellurium to harden such galliumarsenide, germanium and silicon optical bodies.

Further, deposition of ZnS and ZnSe with small amounts of tellurium canbe deposited according to the present invention also by chemical vapordeposition (CVD).

Although only preferred embodiments have been specifically described andillustrated herein, it will be appreciated that many modifications andvariations of the present invention are possible, in light of the aboveteachings, within the purview of the following claims, without departingfrom the spirit and scope of the invention.

I claim:
 1. An aircraft optical window resistant to abrasion from impacting raindrops at high speed and being highly transmissive to light at infrared wavelengths, said optical window being comprised of ZnS with between 0.05% and 2% atomic weight of Te incorporated therein.
 2. An aircraft optical window resistant to abrasion from impacting raindrops at high speed and being highly transmissive to light at infrared wavelengths, said optical window being comprised of ZnSe with between 0.05% and 2% atomic weight of Te incorporated therein.
 3. An aircraft optical window resistant to abrasion from impacting raindrops at high speed comprised of an optical body and a protective layer of ZnS with between 0.05% and 2% atomic weight of Te incorporated into said layer of ZnS with an adhesion layer disposed between the protective layer and the optical body.
 4. The aircraft optical window of claim 3 including an external adhesion layer of ZnS over said protective layer for aiding adhesion of additional layers over the protective layer.
 5. The aircraft optical window of claim 3 wherein the adhesion layer consists of thorium (IV) fluoride.
 6. An aircraft optical window resistant to abrasion from impacting raindrops at high speed and being highly transmissive to light at infrared wavelengths comprising a layer selected from the group of ZnS and ZnSe having incorporated therein Te in a range of 0.5% to 2% atomic weight.
 7. The aircraft optical window of claim 6 wherein the Te therein approximates 1% atomic weight.
 8. An aircraft optical window resistant to abrasion from impacting raindrops at high speed comprised of an optical body and a protective layer, said optical body being selected from the group consisting of glass, ZnS, ZnSe, germanium, gallium arsenide and silicon, the protective layer comprising a layer of ZnSe with between 0.05% and 2% atomic weight of Te incorporated therein.
 9. The aircraft optical window of claim 8 further including an adhesion layer of thorium (IV) fluoride disposed between the optical body and the protective layer.
 10. An aircraft optical window resistant to abrasion from impacting raindrops at high speed comprised of an optical body and a protective layer wherein the optical body is selected from the group consisting of glass, ZnS and ZnSe, and wherein the protective layer is selected from the group consisting of ZnS and ZnSe, the protective layer having between 0.05% and 2% atomic weight of Te incorporated therein and having an adhesion layer disposed between the protective layer and the optical body.
 11. The aircraft optical window set forth in claim 10 wherein the adhesion layer is selected from the group consisting of ZnS and ZnSe.
 12. An aircraft optical window resistant to abrasion from impacting raindrops at high speed comprised of an optical body and a protective layer wherein the protective layer is selected from the group consisting of ZnS and ZnSe, the protective layer having between 0.05% and 2% atomic weight of Te therein.
 13. An aircraft optical window resistant to abrasion from impacting raindrops at high speed comprised of an optical body, a protective layer selected from the group consisting of ZnS and ZnSe and having between 0.05% and 2% atomic weight of Te therein, and an adhesion layer between said optical body and said protective layer selected from the group consisting of ZnS and ZnSe. 